Carbon sequestration is a viable alternative to reduce the emissions of the greenhouse gas carbon dioxide (CO2) from large point sources. Such sequestration holds the potential to provide deep reductions in greenhouse gas emissions. In general, carbon sequestration is a two-step process where the capture of CO2 from a gas stream is followed by permanent storage. The capture step for CO2 represents a major cost in the overall process.
Of particular interest for CO2 sequestration are power generation point sources that use fossil fuels. Since nearly one-third of the anthropogenic CO2 emissions are produced by these facilities, conventional coal-burning power plants and advanced power generation plants—such as integrated gasification combined cycle—present opportunities where carbon can be removed then permanently stored. At the current time, pulverized coal-fired-base steam cycles have been the predominant electric power generation technology. These will continue to be used in the near future. Technologies for capturing CO2 will need to be applied to new more efficient coal-fired facilities and will need to be retrofitted onto existing plants.
Amine-based solid sorbent methodologies which are both effective sequestering agents and economically feasible are needed for CO2 capture from a gaseous mixtures, whether the capture occurs in combustion or gasification power generation systems from flue gas, or in other applications such as natural gas sweetening. Because of the high concentration of CO2 in any of these feed streams, a large quantity of the gas will react with the sorbent and thus produce considerable amounts of exothermic heat. This heat must be removed from the sorbent to prevent temperature instability within the reactor, to assure the sorbent will operate at optimum temperature, and to eliminate the potential degradation of the sorbent because of high temperature excursions.
For coal-fired power plants, the conventional scrubbing system that is currently the comparative baseline for all other capture technologies is monoethanolamine (MEA) scrubbing. This wet scrubbing process removes CO2 in an absorber then regenerates the spent scrubbing liquor in a vessel by indirectly heating the solution with plant steam. Although there have been large scale commercial demonstrations of this technology, the process has several disadvantages including a high heat of reaction, low working capacity, corrosiveness of the solution, the susceptibility of being poisoned, and most notably, its need to be in an aqueous solution. This latter disadvantage results in a large energy need to regenerate the spent solution, especially the sensible heating of the water, which is a minimum of 70 wt % of the solution. The water is recognized as an inert carrier between the absorption and regeneration steps. Another energy loss while regenerating the spent MEA solution includes evaporative heat loss of vaporizing liquid water.
One CO2 capture technology that can be applied to various gas streams has, as a basis, dry regenerable solid sorbents. Examples of these types of sorbents are zeolites, activated carbon, alkali/alkaline earth metals, immobilized amines, metal organic framework, etc. A specific sorbent technology that shows significant advancement are amine-based solid sorbents, such as Basic Immobilized Amine Sorbents (hereinafter BIAS). BIAS consist of amines (primary, secondary, tertiary, or a combination thereof) deposited onto a porous support. The manner of deposition can be random or structured deposition of the amine onto this support (silica, polymer, etc.). When used in the industrial setting, the dry solid sorbent process may act in a similar fashion to the wet scrubbing process in that the sorbent would be transported between an adsorption step and a regeneration step and in that the sorbent is regenerated by a temperature-swing application.
One of the main benefits in using the solid sorbent is the elimination of the sensible heat for the liquid water as compared to MEA. A secondary benefit lies in the lower heat capacity for the solid versus the liquid solvent, also serving to lower the sensible heat required. More CO2 can be adsorbed on a weight or volume basis with the amine-based solid sorbents, so the sorbent system is capable of a significant decrease in the heat duty for the regeneration step. A lower cost of energy service for process involving BIAS as compared to amine wet scrubbing may also result. Thus amine-based solid sorbents have the capability to improve the overall energetics of CO2 capture.
Unfortunately, reactor designs which are amenable to flowing solid sorbents present issues with management of those mobile solid sorbents. For example, sorbents of a particle size capable of efficient CO2 adsorption are often easily aerosolized, carried into a flue stream, and progressed further through the reactor system where they cause damage to downstream components and are overall lost. Sorbent particles of sufficient size to not be at risk for being aerosolized are significantly less efficient at sorption per unit mass, which leads to an increase in the mass of sorbent required. Further, sorbent particles themselves are vulnerable in industrial processes as they do not have the structural integrity necessary for prolonged use in reactors. Where the sorbent has low structural integrity and readily breaks down, greater material investment is required and the sorbent becomes less economical to utilize over other competing materials and methods.
BIAS and their associated processes are among the most widely studied solid sorbents to mitigate post-combustion CO2 emissions. BIAS are organized into three classes (1-3) according to their preparation procedure and amine immobilization mechanisms. Class 1 BIAS are generally prepared by dry or wet impregnation of a support, namely different grades of silica, with a polyamine/hydrophilic solvent (methanol, ethanol, etc.) mixture. Principal polyamines employed are tetraethylenepentamine (TEPA), polyethylenimine (PEI), and generally various linear or branched polyamines that possess different ratios of —NH2 (primary)/—NH (secondary)/—N (tertiary) amine groups that can potentially adsorb CO2. These polyamines are bound to the supports by Si—OH—NH2 hydrogen bonding and also ionic SiO−—NH2+/—NH+ interactions. Primary and secondary amines can capture CO2 under dry and wet conditions while tertiary amines primarily capture CO2 only under humid conditions. The manner of amine deposition on the support can be random or structured deposition of the amine onto the support. In addition to silica, other supports may include clays, polymers, activated carbons, zeolites, and others.
Class 2 BIAS are typically prepared by wet impregnation of a mixture of a reactive aminosilane and anhydrous hydrophobic solvent, usually toluene, onto a dry, pre-treated silica support. Strict control of the H2O content within the system is maintained to manipulate the subsequent grafting reaction between the aminosilane and the silica support. The grafted aminosilanes are immobilized to the silica support via covalent Si—O—Si linkages. These Si—O—Si linkages are also responsible for immobilizing the aminosilane within the bulk of the pore via polymerization.
BIAS sorption capacity is typically calculated either on a weight-percent-of-sorbent basis or mmol CO2/g-sorbent basis. For weight percent basis, the weight of adsorbed CO2 is divided by the weight of sorbent and multiplied by 100. For the mmol CO2/g-sorbent basis, the weight of adsorbed CO2 is divided by the molecular weight of CO2 (44 g/g-mole), multiplied by 1,000, and divided by the sorbent weight.
Advancements in reactor design from batch, fixed-bed systems to continuous circulating fluidized bed, rotating disk, and moving bed systems, and development of a steam-stable sorbent under practical conditions are promising milestones towards commercialization. However, the aforementioned inherent difficulties in the application of such a small particle-size sorbent to industry scale processes remain. For example, BIAS degrades structurally over time as the material is moved from one industrial environment to another. Additionally, the light BIAS can be picked up by and carried into a gas stream, leading to loss of the material and degradation of components downstream. Further, the current amine based sorbent technology utilized in CO2 separation is that the impregnated liquid amines of the BIAS sorbents are vulnerable to leaching from the sorbent pores by condensed steam during practical CO2 adsorption-desorption testing under humidified conditions. The deleterious effect of steam on the CO2 capture of BIAS materials is widely seen in the literature, and was attributed to, in part, amine leaching from the sorbents. Additional difficulties with small particle sorbents include high energy costs to overcome large pressure drop across sorbent beds and failure of, specifically, internal moving parts (valves, conveyors, etc.) by agglomerated or aerosolized particles.
Because of these issues, a composition comprising a BIAS sorbent slurry comprising BIAS suspended in a non-aqueous fluid carrier is advantageous for its improved large scale application. It would be advantageous to provide a sorbent slurry for CO2 capture using an amine-based solid sorbent suspended in a nonaqueous fluid, where the sorbent slurry is capable of efficient CO2 sorption while preventing water loading. Such a method utilizing the slurry further would present a relatively low economic burden when compared to solid based capture systems incorporated into current power generation operations, being relatively easy to incorporate into established power plants when compared to solid based post-combustion separation systems. Thus, utilization of a BIAS slurry increases in CO2 capture capability while minimizing energy and infrastructure requirements is realized.
Accordingly, it is an object of this disclosure to provide a composition for the separation of CO2 from a gaseous mixture, the composition comprising a slurry of BIAS suspended in liquid polymerized siloxane. Further, the object of this disclosure is to provide a method of use for the composition, the method comprising contacting the gaseous mixture with the slurry composition Basic Immobilized Amine Sorbent suspended in liquid polymerized siloxane such that CO2 absorbs into the slurry to form a laden slurry, and regenerating the laden slurry to remove the absorbed CO2.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.